All glass peripherally multi-arcuate disc laser

ABSTRACT

A rectangular-disc laser structure of all-glass support construction. A segmented neodymium-doped glass laser rod is supported in a glass tubing and is designed to permit fluid coolant flow within the tubing and amongst the rod segments. The non-laserable glass supporting means consists of samarium-doped cladding glass to reduce interference by &#39;&#39;&#39;&#39;off axis&#39;&#39;&#39;&#39; spontaneously emitted light. The fluid coolant has an index of refraction which matches that of the laser glass to achieve a high efficiency. The absence of metallic supporting means minimizes the chance of metallic decomposition under the influence of pump light. A peripherally multi-arcuate laser structure utilizing a four-flashtube closewrap is disclosed where part of each flashtube water jacket surface mates with one of the multi-arcuate tubing surfaces.

0 United States Patent [151 3,

Young 51 Nov. 14, 1972 ALL GLASS PERIPI-IERALLY MULTI- 3,628,172 12/1971Matovich ..33l/94.5

ARCUATE DISC LASER Primary Examiner-William L. Sikes 72 l t nven orCharles G Young, Storrs, Conn Atmmey wflhm C Nealon at a! [73] Assignee:American Optical Corporation,

Southbridge, Mass. [57] ABSTRACT [22] Filed: Nov. 11, 1971 Arectangular-disc laser structure of all-glass support construction. Asegmented neodymium-doped glass [21] Appl' 197893 laser rod is supportedin a glass tubing and is designed R l t A to permit fluid coolant flowwithin the tubing and ea pphcauon Dam amongst the rod segments. Thenon-laserable glass '[63]' Continuation-impart of Ser.'No:20,946, Marchsupporting means consists of Samarium-doped 19,1970. cladding glass toreduce interference by off axis" spontaneously emitted light. The fluidcoolant has an [52] us. Cl ..331/94.5, 330/43 index f r f action whichmatches that f the laser 51 Int. (:1 .3015 3/06 glass to achieve a highefficiency The absence of 58 Field of Search ..331/94.5; 330/43; 250/199metallic Supporting means minimizes the chance of metallic decompositionunder the influence of pump [56] References Cited light. A peripherallymulti-arcuate laser structure utilizing a four-flashtube closewrap isdisclosed where UNITED STATES PATENTS part of each flashtube waterjacket surface mates with 3,487,330 12/1969 Gudmundsen ..331/94.5 themulnarcuate mbmg surfaces 3,602,836 8/1971 Young ..331/94.5 7 Clains, 4Drawing Figures ALL GLASS PERIPHERALLY MULTl-ARCUATE DISC LASER Thispatent application is a continuation in part of co-pending patentapplication Ser. No. 20,946 filed on Mar. 19, 1970.

BACKGROUND OF THE INVENTION This patent application is a continuation inpart of co-pending patent application Ser. No. 20,946 filed on Mar. 19,1970.

The instant invention relates to glass lasers and, more particularly, toan all-glass multi-arcuate disc laser structure.

A laser (light amplification by stimulated emission of radiation) is awell-known device consisting of a rod of lasering material betweenparallel, end mirrors, one of which provides full reflection and theother partial reflection and partial transmission of light therethrough.Pump light is introduced into the laser material, generally normal tothe longitudinal axis of the rod between the two end mirrors. The laserlight energy is produced in the laser rod by photonic emission fromactive or high energy level ions in the body of the laser material, withthe pump light increasing the number of ions from lower energy level tothe upper energy level. The pumping light energy abnormally increasesthe upper level population of ions and concomitantly depletes the lowerlevel population of ions creating an inversion of energy states. Some ofthe ions in the upper energy level undergo a spontaneous light emissivetransmission to the lower level, and a portion of the spontaneouslyemissive light reflects back and forth between the mirrored surfacesstimulating similar light emissive transmissions from other upper levelions. As the stimulated emission reflects back and forth repeatedlythrough the rod a sufficiently high intensive pulse of laser lightenergy is emitted for transmission through the partially reflectivesurface.

There are various problems associated with producing a laser beam. Theamount of pumping illumination required to produce laser action inneodymium glass is about 50 watts per cubic centimeter. This pumpinglight produces heat in the lasering material, as does operation at highrepetition rates. Special precautions must be taken for removing thisheat. It is known in the art that temperature rises in the laseringmaterials of glass must be kept uniform to within about 1C. or less inorder for the Fabry-Perot cavity amplification to take place withoutloss of efficiency.

When operating solid glass laser rods at high repetition rates twoproblems appear. First, the rod may exhibit a planar radial split orbreak when the surface tangential stress exceeds the tensile limit ofthe glass. The second problem area is that of change in index ofrefraction with temperature, and in the case of a cylindrical glasslaser rod a strong positive lens effect under moderate average-poweroperating conditions.

For example, the changes in temperature in a nonsegmented laser rodcause an unequal index of refraction radially in the rod because of thelinear expansion of the material. These temperature changes togetherwith the change of index of refraction with temperature at constantdensity, and stress-induced birefringence, produce an induced lenseffect in the rod which is deleterious.

To solve the problems related to excessive heat in the solid laser rod,an initial approach to a solution was a longitudinal sectioning whichyielded a bundle of fagot of smaller-diameter cylindrical laser rods.This approach allowed a considerable improvement in the attainableaverage power in the so-called long-pulse mode of operation, sinceself-focusing does not occur in this mode.

In the so-called Q-switched mode, however, thermal lensing is still aproblem as well as damage at the output end edges of the rods. Inaddition, aligning the plurality of rods is difficult.

A better approach to the problem and that used in the present inventionis to form a multi-arcuate disc laser wherein the cylindric rod issectioned transversely into a number of discs. There are a number ofadvantages in doing this. First, the minimum dimension of each piece canbe made small enough to eliminate thermal splitting. Second, the thermalgradient is now parallel rather than transverse to the laser beam sothat thermal lensing is much reduced. Third, even for a given residualradial thermal gradient, and this can be further minimized by use of anedge cladding, the induced lens power is about an order of magnitudeless than that for'a'rod'. Fourth, the full aperture is usable, comparedto the case of the fagot laser where such is not true, with no rod edgesin the laser beam. Fifth, op tical correction can be applied to eachdisc, if needed. Sixth, discs can be selected for their durability andcomposition as a function of axial position. Seventh, alignment problemsare equivalent to those for a single rod and therefore simpler than fora fagot array. And finally, the multi-arcuate shape of the structureprovides extremely close-wrap optical coupling between four pumpingflashlarnps and the laser discs.

As further background in this particular laser rod art I make referenceto the segmented lasers of the type disclosed in my copendingapplications Disc Laser Modification, Ser. No. 812,119 and Disc LaserVariation, Ser. No. 809,641 assigned to the same assignee as that of thepresent invention.

The disc laser approach, however, has been found to have somelimitations. Since the laser light passes through a considerable lengthof the cooling fluid, this fluid must be transparent and remain soduring prolonged operation. In addition, inter-facial optical lossesbetween the fluid and the discs should be no more than for a solid rod.Also, appropriate mounting means must be employed which will notcompromise laser efficiency, robustness, reliability, etc.

One of the problems involved with disc lasers, and which my inventionsolve, is the efficiency loss resulting from support-metal decompositionunder the influence of pump light. Pump light, for example, from a Xenonflash tube, is intense energy. This light will vaporize the metal whichthen condenses on relatively cooler objects in the vicinity. Metaldeposition on the laser glass, flash tube reflectors and other internalparts decrease pumping efficiency, lasing efficiency, laser life, fluidtransparency, and may cause blockage of coolant flow, and otherproblems.

A solution to the decomposition problem is to eliminate all metal partswhich may come under the influence of pump light. This includes metalsupport or mounting devices holding individual discs. My inventionprovides means for supporting the discs in an allglass construction. Theglass is transparent to pump light and chemically inert to the chosencoolant. Properly selected glass does not decompose under the influenceof pump light. My support arrangement does not compromise pumpingefficiency, lasing efficiency, life or coolant flow.

Therefore, it is an object of my invention to provide an improveddisc-laser structure.

An additional object of my invention is to provide a disc laserstructure not subject to metal decomposition under the influence of pumplight.

A further object my invention is to provide a peripherally muIti-arcuatedisc laser'structure allowing four-flashtube closewrap.

BRIEF DESCRIPTION OF THE INVENTION My invention relates to means formounting discs of laser glass to form a laser device. The laser glassdisc may be circumferentially encompassed by a cladding glass, (thecircumferential glass need not be cladding glass), the periphery ofwhich is multi-arcuate. The assembly of the two types of glass is termeda plate hereafter. One web, or portion of the cladding glass locatedbetween two adjacent flashtubes, contains an aperture, and the two websadjacent thereto contain glass protuberances used for spacing between afirst plate and a longitudinally adjacent one. The aperture is a conduitfor coolant flow across faces of the discs in response to pumping andcooling means external to the laser device. The edges of the claddingbounding the aperture can be rounded to ensure smooth, laminar coolantflow.

The plates are held close-fitted within a glass tube. The tube isarranged to precisely align the plates along a common longitudinal axis.The plates have sufficient thickness to permit alignment by the tube.The plate orientation within the tube is such that the longitudinallyadjacent plates have their apertures diagonally opposite each other, toprovide a coolant flow path across the surface of each plate. Thuscoolant flow is along a tortuous and generally unulating path. Thecoolant fluid is selected to have an index of refraction equal to thatof the selected laser glass, for example, with neodymium-doped glassequal to 1.51 for optimum operation.

Flash tubes used to supply the pump light are each close-fitted and partof each flashtube water jacket surface is contiguous with one of theoutside arcuate surfaces of the glass tubing. Therefore the flashtubesare separated by tubing wall thickness from the active laser medium.

DRAWINGS FIG. 1 is a side elevation in partial section, partially brokenaway of an embodiment of my present invention viewed perpendicularly tothe longitudinal axis.

FIG. 2 is a cross-sectional view of the arrangement of FIG. 1 takenalong the line 22.

FIG. 3 is a view of an alternative embodiment.

FIG. 4 is diagrammatic end-view of the preferred embodiment of mypresent invention.

DETAILED DESCRIPTION FIG. 1 is an embodiment of my invention. Glasslaser plates 11 are aligned along longitudinal axis 21 by rectangularglass tubing 19 into which plates 1 1 are inserted in a close tolerancefit. Glass spacers 13 are raised from the surfaces of plates 11 along alongitudinal direction. The spacers abut adjacent plates 11 and therebyprovide a space for coolant flow 22 between plates. Coolant flow 22 iscontrolled by a pump 17. Flow enters segmented laser device 23 viaorifice 16 and exits via orifice 18 back to pump 17. Details of the pumpand associated cooling equipment are shown schematically herein tomaintain drawing simplicity. More detailed information may be found inU.S. Pat. No. 3,569,860 to Booth. Flow 22 within laser device 23 takesan undulating path through apertures 12 and spaces 14, to provideefficient cooling of plates 1 1. The fluid is chosen to have an index ofrefraction of approximately 1.51 to match that of the preferred approximately 2.4 percent by weight Nd" doped laser glass for maximumefficiency. For the samarium glass I prefer approximately 10 percent byweight dopant. For further information regarding the samarium andneodymium glasses useable according to this invention, I make referenceto U.S. Pat. No. 3,445,785 to Koester et al. assigned to the same.assignee as that of the present invention.

Flash tubes are parallel to the outer wall of glass tubing 19 andalthough only two flash tubes are shown, more may be used. Flash tubes15 provide the necessary excitation energy to the laserable material.The lasering action takes place in a direction parallel to longitudinalaxis 21. The two ends of the laser rod contain parallel mirroredsurfaces 25 and 26, both mirrors being orthogonal to the longitudinalaxis; one surface is completely reflective and the other is partlyreflective and partly transmissive to allow the exiting of the laserbeam. As is well understood, the mirrors reflect light energy back andforth through the laser structure to promote light amplification.

Laser-disc can be seen in FIG. 2 as a circle. The encompassing andsupporting glass ring 31 is of samarium-doped glass. It absorbs anyoff-axis light rays which could interfere with the laser beam. Anaperture 12 is formed through glass ring 31. Glass spacers 13 are shownin corners adjacent to the corner through which aperture 12 is formed.Water-cooled reflector is provided which combined with wall 32 forms aconduit 34 for water coolant (not shown). Ultraviolet light absorbingglass flash tube water jacket 33 is provided which combines with thesurface of the flash tube to form another conduit 36 for water coolant(not shown).

It is seen that glass support construction is used throughout. No metalparts are used internal to the glass tubing 19.

FIG. 3 is a view of an alternative embodiment of my present invention.Glass tubing 40 is hexagonal in cross-section and is composed ofsamarium-doped cladding glass. The tubing 40 serves two purposed herein:to absorb any off-axis light; and to channel the coolant flow. Thehexagonal cross-section permits usage of a maximum of six flash tubes.Plate 41 is the laser material. Spacers 42 are also provided. It shouldbe understood other polygonal and non-polygonal shapes may be used fortubings, if compatible plate cross-sections are used.

An example of a non-polygonal shape is shown in FIG. 4, where afour-flashtube closewrap structure is depicted. Laser disc 57 is seen asa circle and is equivalent to disc 30. Encompassing and supportingmember 56 has a crosssectional shape determined by four peripheralnon-intersecting arches with a web between adjacent arches, and member56 is essentially equivalent to ring 31. Member 56 can be made oflightabsorbing samarium doped glass or can be made from glass withoutdopant. Protuberances 55 are shown located at opposite webs of member56. These protuberances or spacers are equivalent to glass spacers 13.Hollow tubing 54 in this embodiment has a four-arc shape. Multi-arcuateis intended to indicate that less than and more than four arcs may beused. Tubing 54 is arranged to internally precisely align members 56 andto externally mate and be contiguous with part of the outside surfacesof each of four flashtube water jackets 51 as shown. Flashtube 53 issurrounded by water coolant contained in conduit 52 between water jacket51 and flashtube 53. Reflector 58 is arranged to reflectrearwardly-radiated light from each flashtube and to concentrate thelight at laser discs 57. Aperture 59 is equivalent to aperture 12.

The operation of a laser system with this shaped structure isessentially equivalent to that described above, where the improvement ofFIG. 4 provides at least a greater laser pumping efficiency. Thus,operative description will not be repeated here.

Tubing 54 can be precision bore tubing which is drawn over a mandrel,such as TRUBORE. The outer dimension can be ground slightly for goodfits. Member 56 can be cut octagonally and precision polished to createfour arcuate shapes and to provide a good fit with flashtube waterjacket 51. However, an initial octagonal cut is not necessary; the webscan be formed to extend between the flashtubes to a greater extent thanthat shown. Member 56 can be recessed at diametrically opposite webs toaccept spacers 55. Spacers 55 might be glass or non-reactive plasticsuch as teflon or one of the RTVs. The latter would provide resiliencein the presence of expansion of the structural pieces.

Disc 57 can be normal or at near Brewsters angle to the beam. In aparticular instance, discs 57 can be a few millimeters thick with a fewtenths of a millimeter spacing between discs. A typical flashtube, anEGG FX-67, has been used for which a diameter of 18 millimeters for disc57 appears to be optimum.

It is to be understood that other laser materials can be utilized. Laserglass of dopants other than neodymium and coated laser glass can beemployed.

It is to be further understood that the plates need not necessarily bealigned to form right angles with the longitudinal axis. For example,they could be aligned at the Brewsters angle.

It is to be further understood that four flashlamps need not be used.Two or more flashlamps can be used with appropriate peripheral arcuatedesign. Also, the tubing need be doped and need not be glass;the tubingcould be plastic. If the tubing is doped glass, the dopant could besamarium or cerium.

From the embodiments of my invention disclosed herein, it is understoodthat other changes can be made without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:

1. A light-transparent laser structure comprising linear glass tubing ofmulti-arcuate crossection said tubing arranged to contain flow of afluid, a plurality of glass plates, each of said plates containing atleast a portion of laser material and being close fitted within theinside surface of said multi-arcuate tubing and arranged to span thehollow of said tubing, there being an aperture near an edge of each ofsaid plates which are substantially equally spaced from each other,sequential plate apertures arranged to cause tortuous flow of said fluidthrough said tubing, said flow arranged to cool each of said plates, anda plurality of flashtubes externally adjacent said tubing, a portion ofthe surface of each of said flashtubes arranged to be fittedcontiguously with a portion of the external surface of saidmulti-arcuate tubing for providing close-wrap, efi'icient, lightenergization to said plates.

2. A laser structure as recited in claim 1 wherein said plurality offlashtubes is four.

3. A laser structure as recited in claim 2 wherein each of said platescomprises a disc of laser glass and an apertured glass member havingfour webs, said member encompassing and supporting said disc within saidtubmg.

4. A laser structure as recited in claim 3 wherein said disc of laserglass is neodymium-doped glass.

5. A laser structure as recited in claim 1 wherein said tubing issamarium doped glass.

6. A laser structure as recited in claim 3 wherein said member issamarium doped glass.

7. A laser structure as recited in claim 1 wherein said fluid is aliquid with an index of refraction of approximately 1.51.

n: m It

1. A light-transparent laser structure comprising linear glass tubing ofmulti-arcuate crossection said tubing arranged to contain flow of afluid, a plurality of glass plates, each of said plates containing atleast a portion of laser material and being close fitted within theinside surface of said multiarcuate tubing and arranged to span thehollow of said tubing, there being an aperture near an edge of each ofsaid plates which are substantially equally spaced from each other,sequential plate apertures arranged to cause tortuous flow of said fluidthrough said tubing, said flow arranged to cool each of said plates, anda plurality of flashtubes externally adjacent said tubing, a portion ofthe surface of each of said flashtubes arranged to be fittedcontiguously with a portion of the external surface of saidmulti-arcuate tubing for providing close-wrap, efficient, lightenergization to said plates.
 2. A laser structure as recited in claim 1wherein said plurality of flashtubes is four.
 3. A laser structure asrecited in claim 2 wherein each of said plates comprises a disc of laserglass and an apertured glass member having four webs, said memberencompassing and supporting said disc within said tubing.
 4. A laserstructure as recited in claim 3 wherein said disc of laser glass isneodymium-doped glass.
 5. A laser structure as recited in claim 1wherein said tubing is samarium doped glass.
 6. A laser structure asrecited in claim 3 wherein said member is samarium doped glass.
 7. Alaser structure as recited in claim 1 wherein said fluid is a liquidwith an index of refraction of approximately 1.51.